A variety of communication platforms, such as an unmanned aerial vehicle (UAV)-mounted system diagrammatically illustrated at 10 in FIG. 1, are required to maintain effectively continuous broadbeam communication capability (with a ground station 12) without having to (physically or electronically) steer the aerial system's antenna 14. Because both the range and direction of the aerial vehicle-mounted system, relative to the ground station, are dynamic, it is essential that the airborne equipment's antenna 14 provide communication coverage that is at least hemispheric. The antenna should provide somewhat `above the horizon` coverage, and be designed for circular polarization, in order to accommodate changes in aircraft attitude (roll, pitch and yaw). In addition, because of the significant reduction in signal strength, increased probability of multipath and rain fades at the horizon, especially at X band and higher frequencies, it is preferred that the antenna's radiation/directivity pattern exhibit peak gain at or in the vicinity of the horizon.
Unfortunately, existing antenna architectures address only subsets of these requirements. For example, as diagrammatically shown in FIG. 2, a biconical antenna 20 exhibits a very narrow, flat pattern 21, which has a peak gain 22 at the horizon, and is therefore potentially well suited for long range, reduced elevation look angle coverage. Unfortunately, the gain over the remainder of the characteristic drops off very rapidly from the horizon peak and exhibits a null or close to a null over a very substantial portion of coverage on either side of nadir 23 (looking straight down). Even though relatively low gain can be tolerated at nadir, the very significant reduction in gain exhibited by a biconical antenna over a wide portion of intended coverage between nadir and the vicinity of the horizon is not acceptable. A further drawback to a biconical antenna is the need for an external polarizer.
A bifilar helical configuration, such as diagrammaticallly shown at 30 in FIG. 3, on the other hand, has a relatively wide beam radiation pattern 32, which exhibits significant gain not only at and in the vicinity of the horizon 33, but also over a major coverage look angle that is well displaced from the horizon. However, a major drawback to a bifilar helix configuration is the fact that it has a poor axial ratio for circular polarization. In addition, the upper end of the performance bandwidth of bifilar helical antennas is limited to the neighborhood of 20-25 GHz.
Other conventional antenna architectures that have been proposed for non-steered broad coverage (UAV) applications include circular dipoles (which suffer the same limitations as the biconical approach), patch antennas (which have a null at the horizon), and slot arrays (which suffer reduced gain toward the horizon, require an external polarizer and have unproven performance). A further problem of each of the above conventional approaches is the fact that the antenna pattern cannot be shaped as necessary to provide optimal coverage for a particular application.